The present invention relates to a system for providing an alarm message for maintenance personnel when a temperature exceeding a specified level is experienced in a refrigeration unit. In accordance with the present invention, the part of the system in the location of the refrigeration unit is compact and employs no permanent wiring. Preferably, radio transmission of an alarm signal to a remote location permits location of heavier equipment of the system in that remote location from which pre-recorded alarm messages can be transmitted.
Prior art systems have measured an inferred indication of temperature, rather than temperature itself. Such systems have employed devices which sense vacuum, freon pressure, operable time of the refrigeration compressor, motor and other portions of the refrigeration unit to determine when the refrigeration unit loses its cooling capacity. Transmitters at the location of a sensor transmitting a radio frequency signal representative of sensed information have been known. Furthermore, automatic telephone dialers responsive to a signal from a remote transmitter to dial a pre-programmed number, and a message playing unit to play a pre-programmed alarm message over the open telephone line dialed have been known. Temperature sensing switches of the type preferred by the applicant have been suggested by H. Gjerken in his U.S. Pat. No. 3,148,363, for fire alarm systems. However, the systems involved are relay systems which would be objectionable in a super market, for example, because of bulk and probably because of the requirement of making permanent wiring.
Also, monitoring systems for monitoring refrigerated display cases have been suggested in the prior art, such as by Norman B. Bergeron in U.S. Pat. No. 3,594,749. However, there is no commercial device available for this purpose. The Bergeron system is also like other known prior art in that it provides no remote signal. While Bergeron shows a plurality of systems with a central cabinet there is no suggestion of remoteness and apparently Bergeron intends that the apparatus shall be used directly at the location of the refrigeration unit. The alarm is a light which must be observed by someone, presumably either on the scene, or more likely, periodically monitoring the location.